The present invention relates to a system for controlling idle speed of an automotive engine through a feedback control operation for causing the actual idle speed to converge to a desired idle speed.
In an idle speed control system, a bypass having an idle speed control valve (ISCV) is provided around a throttle valve of the engine. The idle speed control valve is operated to control the amount of intake air for causing the actual idle speed to converge to the desired idle speed by a feedback control system.
In the feedback control system, a dead zone is provided with respect to the desired idle speed in order to prevent the opening degree of the ISCV from hunting when the idle speed approximates the desired idle speed. When the engine speed is within the dead zone, the feedback operation is stopped so that the opening degree of the ISCV is held constant Hence, the convergence of the idle speed is improved.
Japanese Patent Application Laid-Open 59-226250 discloses such an idle control system where the feedback operation is carried out only in a transient state, such as the time when an air-conditioner is started and stopped, which causes fluctuation of the idle speed.
In the feedback control operation, a dead zone is provided.
In a no load state, the engine speed is controlled by an open-loop control. Accordingly, there are disadvantages in that the idle speed can fluctuate without converging to a desired speed.
Furthermore, the width of the dead zone is obtained by adding a predetermined value to the average of the differences of peak engine speeds at the time of fluctuation. Since the width of the dead zone changes with the fluctuation of the engine speed, it is difficult for the engine speed to accurately converge to the desired idle speed. More particularly, the width of the dead zone for the desired idle speed (for example 800 rpm) is set at a value (for example .+-.50 rpm) which is slightly larger than the fluctuating width of the engine speed. Thus, an undesirable feedback operation caused by the fluctuation of the engine speed is avoided. However, since the dead zone is wide (750-850 rpm), the range of the controlled engine idling speed becomes large. To the contrary, if the dead zone is reduced, the ISCV is operated even if a slight fluctuation of the engine speed occurs. As a result, the idling speed fluctuates.